EGW saw the need for a US made, mid-tier scope ring. One that was versatile, robust, AND also a great value. They weren't happy with the current options on the market, so they created their own. Introducing the Keystone Series scope rings. Improvements: Machined in house in one of EGW's CNC machining centers 6061 aircraft grade aluminum T15 mounting hardware for the ring caps Chromoly crossbolt and A 1/2" nut for reliable and repeatable engagement Fits picatinny and weaver style rails Horizontally split design allows for easy installation EGW rings are measured from where the ring sits on the top of the base, to the cente Mfg: Egw